1,986 research outputs found
Ternary and quaternary oxides of Bi, Sr, and Cu
Before the discovery of superconductivity in an oxide of Bi, Sr, and Cu, the system Bi-Sr-Cu-O had not been studied, although several solid phases had been identified in the two-component regions of the ternary system Bi2O3-SrO-CuO. The oxides Sr2CuO3, SrCu2O2, SrCuO2, and Bi2CuO4 were then well known and characterized, and the phase diagram of the binary system Bi2O3 -SrO had been established in the temperature range 620 to 1000 C. Besides nine solutions of compositions Bi(2-2x) Sr(x) O(3-2x) and different symmetries, this diagram includes three definite compounds of stoichiometries Bi(2)SrO4, Bi2Sr2O5, and Bi2Sr3O6 (x = 0.50, 0.67 and 0.75 respectively), only the second of which with known unit-cell of orthorhombic symmetry, dimensions (A) a = 14.293(2), b = 7.651(2), c = 6.172(1), and z = 4. The first superconducting oxide in the system Bi-Sr-Cu-O was initially formulated as Bi2Sr2Cu2O(7+x), with an orthorhombic unit-cell of parameters (A) a = 5.32, b = 26.6, c = 48.8. In a preliminary study the same oxide was formulated with half the copper content, Bi(2)Sr(2)CuO(6+x), and indexed its reflections assuming an orthorhombic unit-cell of dimensions (A) a = 5.390(2), b = 26.973(8), c = 24.69(4). Subsequent studies by diffraction techniques have confirmed the composition 2:2:1. A new family of oxygen-deficient perovskites, was characterized, after identifying by x ray diffraction the phases present in the products of thermal treatments of about 150 mixtures of analytical grade Bi2O3, Sr(OH)2-8H2O and CuO at different molar ratios. X ray diffraction data are presented for some other oxides of Bi and Sr, as well as for various quaternary oxides, among them an oxide of Bi, Sr, and Cu
Dynamical stability criterion for inhomogeneous quasi-stationary states in long-range systems
We derive a necessary and sufficient condition of linear dynamical stability
for inhomogeneous Vlasov stationary states of the Hamiltonian Mean Field (HMF)
model. The condition is expressed by an explicit disequality that has to be
satisfied by the stationary state, and it generalizes the known disequality for
homogeneous stationary states. In addition, we derive analogous disequalities
that express necessary and sufficient conditions of formal stability for the
stationary states. Their usefulness, from the point of view of linear dynamical
stability, is that they are simpler, although they provide only sufficient
criteria of linear stability. We show that for homogeneous stationary states
the relations become equal, and therefore linear dynamical stability and formal
stability become equivalent.Comment: Submitted to Journal of Statistical Mechanics: Theory and Experimen
Relaxation to thermal equilibrium in the self-gravitating sheet model
We revisit the issue of relaxation to thermal equilibrium in the so-called
"sheet model", i.e., particles in one dimension interacting by attractive
forces independent of their separation. We show that this relaxation may be
very clearly detected and characterized by following the evolution of order
parameters defined by appropriately normalized moments of the phase space
distribution which probe its entanglement in space and velocity coordinates.
For a class of quasi-stationary states which result from the violent relaxation
of rectangular waterbag initial conditions, characterized by their virial ratio
R_0, we show that relaxation occurs on a time scale which (i) scales
approximately linearly in the particle number N, and (ii) shows also a strong
dependence on R_0, with quasi-stationary states from colder initial conditions
relaxing much more rapidly. The temporal evolution of the order parameter may
be well described by a stretched exponential function. We study finally the
correlation of the relaxation times with the amplitude of fluctuations in the
relaxing quasi-stationary states, as well as the relation between temporal and
ensemble averages.Comment: 37 pages, 24 figures; some additional discussion of previous
literature and other minor modifications, final published versio
Density-Temperature-Softness Scaling of the Dynamics of Glass-forming Soft-sphere Liquids
The principle of dynamic equivalence between soft-sphere and hard-sphere
fluids [Phys. Rev. E \textbf{68}, 011405 (2003)] is employed to describe the
interplay of the effects of varying the density n, the temperature T, and the
softness (characterized by a softness parameter {\nu}^{-1}) on the dynamics of
glass-forming soft-sphere liquids in terms of simple scaling rules. The main
prediction is that the dynamic parameters of these systems, such as the
{\alpha}-relaxation time and the long-time self-diffusion coefficient, depend
on n, T, and {\nu} only through the reduced density n^\ast \equiv
n{\sigma}^{3}_{HS}(T, {\nu}),where the effective hard-sphere diameter
{\sigma}_{HS}(T, {\nu}) is determined, for example, by the
Andersen-Weeks-Chandler condition for soft-sphere-hard-sphere structural
equivalence. A number of scaling properties observed in recent simulations
involving glass-forming fluids with repulsive short range interactions are
found to be a direct manifestation of this general dynamic equivalence
principle. The self-consistent generalized Langevin equation (SCGLE) theory of
colloid dynamics is shown to accurately capture these scaling rule
Long-Range Effects in Layered Spin Structures
We study theoretically layered spin systems where long-range dipolar
interactions play a relevant role. By choosing a specific sample shape, we are
able to reduce the complex Hamiltonian of the system to that of a much simpler
coupled rotator model with short-range and mean-field interactions. This latter
model has been studied in the past because of its interesting dynamical and
statistical properties related to exotic features of long-range interactions.
It is suggested that experiments could be conducted such that within a specific
temperature range the presence of long-range interactions crucially affect the
behavior of the system
1-d gravity in infinite point distributions
The dynamics of infinite, asymptotically uniform, distributions of
self-gravitating particles in one spatial dimension provides a simple toy model
for the analogous three dimensional problem. We focus here on a limitation of
such models as treated so far in the literature: the force, as it has been
specified, is well defined in infinite point distributions only if there is a
centre of symmetry (i.e. the definition requires explicitly the breaking of
statistical translational invariance). The problem arises because naive
background subtraction (due to expansion, or by "Jeans' swindle" for the static
case), applied as in three dimensions, leaves an unregulated contribution to
the force due to surface mass fluctuations. Following a discussion by
Kiessling, we show that the problem may be resolved by defining the force in
infinite point distributions as the limit of an exponentially screened pair
interaction. We show that this prescription gives a well defined (finite) force
acting on particles in a class of perturbed infinite lattices, which are the
point processes relevant to cosmological N-body simulations. For identical
particles the dynamics of the simplest toy model is equivalent to that of an
infinite set of points with inverted harmonic oscillator potentials which
bounce elastically when they collide. We discuss previous results in the
literature, and present new results for the specific case of this simplest
(static) model starting from "shuffled lattice" initial conditions. These show
qualitative properties (notably its "self-similarity") of the evolution very
similar to those in the analogous simulations in three dimensions, which in
turn resemble those in the expanding universe.Comment: 20 pages, 8 figures, small changes (section II shortened, added
discussion in section IV), matches final version to appear in PR
Maturity related differences in body composition assessed by classic and specific bioimpedance vector analysis among male elite youth soccer players
The aim of this study was to analyze the efficiency of classic and specific bioelectrical impedance vector analysis (BIVA) in the assessment of maturity related differences in body composition among male elite youth soccer players, and to provide bioelectrical impedance reference data for this category. A group of 178 players (aged 12.1 \ub1 1.6 years) were registered in a professional Italian soccer team participating in the first division (Serie A). They were divided into three groups according to their maturity status while bioelectrical resistance and reactance were obtained. The classic and specific BIVA procedures were applied, which correct bioelectrical values for body height and body geometry, respectively. Percentage of fat mass (FM%) and total body water (TBW (L)) were estimated from bioelectrical values. Age-specific z-scores of the predicted age at peak height velocity identified 29 players as earlier-, 126 as on time-, and 23 as later-maturing. TBW was higher (p < 0.01) in adolescents classified as \u201cearly\u201d maturity status compared to the other two groups and classic BIVA confirmed these results. Conversely, no differences in FM% were found among the groups. Specific vector length showed a higher correlation (r = 0.748) with FM% compared with the classic approach (r = 0.493). Classic vector length showed a stronger association (r = 120.955) with TBW compared with specific (r = 120.263). Specific BIVA turns out to be accurate for the analysis of FM% in athletes, while classic BIVA shows to be a valid approach to evaluate TBW. An original data set of bioelectric impedance reference values of male elite youth soccer players was provided
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Phase transitions of quasistationary states in the Hamiltonian Mean Field model
The out-of-equilibrium dynamics of the Hamiltonian Mean Field (HMF) model is
studied in presence of an externally imposed magnetic field h. Lynden-Bell's
theory of violent relaxation is revisited and shown to adequately capture the
system dynamics, as revealed by direct Vlasov based numerical simulations in
the limit of vanishing field. This includes the existence of an
out-of-equilibrium phase transition separating magnetized and non magnetized
phases. We also monitor the fluctuations in time of the magnetization, which
allows us to elaborate on the choice of the correct order parameter when
challenging the performance of Lynden-Bell's theory. The presence of the field
h removes the phase transition, as it happens at equilibrium. Moreover, regions
with negative susceptibility are numerically found to occur, in agreement with
the predictions of the theory.Comment: 6 pages, 7 figure
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